Re: 4BSD TCP Ethernet Throughput


John Carter (retrac@rice.edu)
Thu, 27 Oct 88 11:08:04 CDT


BAD MSG:
takes a long time to recover from interruptions. I hadn't seen too many
ollisions, so I hadn't thought of that, but it seems to fit with
some other observations I've made concerning the interface. The Intel
interface tends to drop packets reasonably frequently when receiving large
packet bursts (blasts), presumably because of its inability to DMA in to
memory fast enough. Another problem I have had, which seems to be caused
by the interface, is that it takes a relatively long time for the
interface to interrupt the processor when an event occurs (either a packet
reception completing, or a transmission completing). An annoying
"feature" of the interface is the fact that you can't have receive buffers
start on odd boundaries (I suppose that they wanted to simplify the DMA
design).

    Finally, despite the great effort put in to designing a "programmable"
interface, I don't really think it was much easier to get to do what we
needed than the LANCE was. True, it has a few less annoying
programmability problems (e.g., a less obscure method of selectively
accepting multicast packets, and the decision to not append the useless
harldware CRC to the end of each packet, thus requiring its handling
[which is particularly painful for optimistic blast, because I don't want
it to get redirected in to user memory, sigh]), but the overall decline in
raw performance overshadows these issues. Heck, you only have to program
the thing once, performance lives forever (and is what counts)!

John

[And for anyone out there reading the cc'd copy of this, I'd like to add
 my voice to the call for a better ethernet interface design. The
 existing ones are quite lacking in many ways, and put far too much
 load on the processor. If you're going to design one, I have some ideas
 for what would be useful. There were some interesting designs presented
 at Sigcomm '88 which address some of the problems I have with current
 designs, though not all of them.]

    Finally, since I cc'd this to tcp-ip, all of the above is
    Copyright (c) 1988 by John Carter. This way, I don't get
    misquoted and, more importantly, Van doesn't get misquoted
    by my references to his work.



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